Tissue contrast

The outstanding advantage of MRI for the anatomical examination of the brain is the easily visible contrast in the images between grey matter, white matter, and cerebrospinal fluid. In particular, contrast between parenchymal tissues (grey and white matter) has made MRI the imaging research tool of choice for identifying subtle cortical abnormalities in a wide variety of psychiatric disorders.

Tissue contrast in magnetic resonance images is determined by differences in the density of protons, and their physical and chemical environment. A tissue that is largely composed of air, like the lungs, will have a lower proton density than a tissue, such as the cerebrospinal fluid, that is composed largely of water, which in turn will have a lower proton density than parenchymal brain tissues. The physicochemical environment of protons has a marked effect on spin-lattice relaxation. If protons are mainly in freely diffusing water molecules, as they are in cerebrospinal fluid, T1 will be prolonged, whereas if they are mainly bound to large macromolecules, as they are in fat, T1 will be short (Table 1). Since grey matter contains proportionally less fat than myelinated white matter, T1 is longer for grey matter. Spin-spin relaxation is likewise determined, in part, by the immediate physical environment of protons in the tissue; liquid tissues will have prolonged T2 times compared with solid tissues. Other effects on apparent relaxation times ( T2*) include minute fluctuations or inhomogeneities in the strength of the external magnetic field, which may be due to the local paramagnetic effects of iron-containing compounds such as haemoglobin.

Table 1 Relaxation times at 1.5 T for different tissue types

The parameters of the spin echo pulse sequence, repetition time (TR), and time to echo (TE), can be judiciously adjusted to acquire images that are sensitive to or weighted by one or other of these possible sources of tissue contrast.

If TR is long (> 1000 ms) and TE is short (< 20 ms), contrast in the images will be weighted by differences between tissues in proton density. Proton-density-weighted images show good contrast between relatively hyperintense parenchymal tissue and hypointense cerebrospinal fluid ( PJate.7).

If TR is short (< 1000 ms) and TE is short (< 20 ms), contrast in the images will be weighted by tissue differences in spin-lattice relaxation. Trweighted images show excellent contrast between hyperintense white matter and relatively hypointense grey matter (Plate.../). For this reason, Trweighted images are widely used to measure quantitative abnormalities in size or shape of the cerebral cortex.

If TR is long (> 1000 ms) and TE is long (> 20 ms), contrast in the images will be weighted by tissue differences in spin-spin relaxation. T2-weighted images show strong contrast between hyperintense cerebrospinal fluid and parenchymal tissues ( Pla.te..Z), unless there is congestion or oedema of the parenchyma, in which case the T2-weighted signal will be increased. For this reason, T2-weighted images are widely used to identify acute, inflammatory, and ischaemic lesions.

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